Grating based multiplexer/demultiplexer component

ABSTRACT

A grating based demultiplexer module is described. The module includes an integrally formed first section, second section, and third section. The first section includes a diffraction grating formed on the surface of the first section and directs a WDM beam onto the internal surface of the diffraction grating. The third section is positioned to receive angularly separated light from the external surface of the diffraction grating. In some embodiments, the third section can direct individualy beams of the angularly separated light onto the surface of optical detectors. The third section provides structural support and maintains the alignment between the first section and the third section.

BACKGROUND

1. Field of the Invention

The present invention relates to a multiplexer or demultiplexer opticalcomponent and, in particular, to a multiplexer or demultiplexer for awavelength division multiplexed optical system.

2. Discussion of Related Art

Wavelength division multiplexing has become a standard in opticalnetworks over the last few decades. Wavelength division multiplexing(WDM) exploits the potential bandwidth of optical fibers by transmittingdata over several channels on the same fiber. Each channel istransmitted on the optical fiber at a different wavelength. The rate ofdata transmission over the fiber, then, can be increased by a factor ofM, where M is the number of channels (i.e., the number of differentwavelengths) being transmitted over the fiber.

Recently, an explosion of WDM technologies has appeared on the market.Systems having 8, 16, and 32 channels have become commonplace. DenseWDM, DWDM, for example, can have 32 channels following an ITU grid with0.8 nm wavelength separation. However, in order to effectively utilizethe bandwidth of the optical fiber, optical signals must be multiplexedand demultiplexed onto the fiber.

In WDM systems, optical signals are transmitted over a set of Mchannels. The M channels are multiplexed at the transmitter so that Mwavelengths of light are simultaneously transmitted on an optical fiberto a receiver system. At the receiver system, the M channels aredemultiplexed into optical signals transmitted at individual wavelengthsof light. The individual wavelengths of light can then be directed tophotodetectors so that the optical signals can be converted intoelectrical signals for processing by subsequent electronic circuitry.

In some demultiplexing systems, an optical fiber can be directlyattached to a dielectric waveguide. The waveguide geometry exploitsinterference and/or diffraction in order to separate differentwavelength constituents of the input light beam. These systems aredifficult to fabricate, have large insertion losses, and are onlyapplicable to single-mode fibers.

Demultiplexing can be accomplished with diffraction gratings, prisms, orfilters, for example. The major problem with such devices is that theyoften include bulky and costly lenses and such which are very hard toreliably align, leading to large manufacturing costs and a bulky finalproduct. Conventionally, most WDM or DWDM demultiplexer systems includefilter-based demultiplexers, primarily do to the fact that formation ofgratings in glass are more expensive to fabricate than are filters.

In the past few years, with the advent of high data rate communications,the concept of wide WDM (WWDM) (channel spacings of ˜25 nm) has beenproposed and is currently being actively considered as a standard for 10Gigabit Ethernet and Fiber Channel communication systems. Currently,demultiplexer systems being proposed for use in WWDM systems haveinvolved filter based demultiplexer systems. However, filter baseddemultiplexer systems are expensive, primarily because of the assemblycosts due to the small filters and multiple other components whichrequire time consuming alignment and assembly. Furthermore, the designof a filter based demultiplexer system is not scalable to systems havingmore channels. Also, the design of the filter based systems cannot bescaled down to small physical beam separation distances due tolimitation on the size of filters and beam clipping. Finally, filterbased demultiplexer systems require complicated alignment of severalsubassemblies.

Therefore, there is a need for a less expensive and more versatiledemultiplexer system for utilization in WWDM, WDM or DWDM systems.

SUMMARY

In accordance with the present invention, a demultiplexer with adiffraction grating for separating the separate wavelengths of light ispresented. A demultiplexer according to the present invention includes agrating, which spatially separates the separate wavelengths of lightrepresenting individual channels on a wavelength grid, integrally formedon a demultiplexer module. The wavelength grid can be any grid,including those grids commonly utilized in WDM, DWDM, or WWDM opticalsystems.

A demultiplexer module according to the present invention includes afirst section, a second section integrally formed with the firstsection, and a third section integrally formed with the first and secondsections. The first section receives a WDM beam and directs the WDM beamonto an internal surface of a diffraction grating integrally formed on asurface of the first section. The diffraction grating, then, has aninternal surface internal to the first section and an external surfaceexternal to the first section. The diffraction grating angularlyseparates beams of individual wavelengths from the WDM beam and providesthose beams at the external surface of the diffraction grating. Thethird section is positioned to receive the angularly separated beams.The second section provides structural support for the demultiplexermodule and also relatively positions the third section with respect tothe first section.

In some embodiments, the first section can also include an internalreflective surface for directing the WDM beam onto the internal surfaceof the diffraction grating. The internal reflective surface, which isinternal to the first section, can rely on total internal reflection or,in some embodiments, can be coated on an external surface of the firstsection opposite the internal reflective surface of the first section toenhance the internal reflection. The coating, for example, can be thinfilms provided to enhance reflection or can be a thin film of gold orsilver backing.

In some embodiments, the first section can further include a collimatinglens to receive the WDM beam from an optical fiber. Further, the thirdsection can further include a focusing lens to couple the angularlyseparated beams onto a detector array or into individually opticalfibers.

A demultiplexer module according to the present invention can be formed,in some embodiments, from an optical plastic, in a single piece,removing any requirements for alignment of separate components of thefirst, second and third sections. Therefore, demultiplexers according toembodiments of the present invention can be inexpensively mass produced.

In some embodiments, the demultiplexer module includes an integrallyformed barrel. The barrel is positioned relative to the first section inorder that the first section receives a WDM beam from an optical fiberpositioned and held by the barrel. A barrel includes a fiber access andfiber stop. An optical fiber positioned in the fiber access at the fiberstop is, then, aligned with the first section of the demultiplexermodule. In operation, an optical fiber that can carry the WDM opticalsignal is positioned into the barrel and the light from the opticalfiber is collimated by the collimating lens. In some embodiments, thebarrel can be formed separately and attached to a post that isintegrally formed with the demultiplexing module.

In some embodiments, the collimated light beam is incident on thediffraction grating substantially normal to the internal surface of thediffraction grating. The focusing lens of the third section, then, ispositioned with respect to the external surface of the diffractiongrating, from which the angularly separated individual beams areemitted, so as to capture a first or higher order diffraction peak fromthe diffraction grating. The focusing lens then focuses the individuallyseparated beams of light from the diffraction grating onto an array ofoptical detectors. Each of the optical detectors, then, detects light ofa particular wavelength. In some embodiments, light from individuallyseparated beams are coupled into optical fibers, which then transmitlight of separate data transmission channels corresponding to theseparate wavelengths of the individual beams. In some embodiments, wherethe module is utilized in a multiplexer, optical detectors may bereplaced with optical sources that emit light of appropriatewavelengths.

In some embodiments, the diffraction grating is a one-dimensional ruledgrating. Some embodiments include a two-dimensional ruled grating. Insome embodiments, the diffraction grating is formed so as to couplesubstantially all of the incident light into the first order diffractionpeak so as to reduce the amount of incident radiation lost to otherorders of diffraction peaks (e.g., the zeroth order).

In some embodiments, a multiplexer system can be formed with thedemultiplexer mounting by coupling light of different wavelengths ontothe transmission grating at the appropriate angle to couple that lightinto an optical fiber. In these embodiments, the optical detectors ofthe demultiplexer can be replaced by optical sources of the appropriatewavelengths to form a multiplexer. These optical sources can be opticalfibers carrying individual channels of optical data at the appropriateoptical wavelengths.

These and other embodiments are further described below along with thefollowing Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A, 1B and 1C show an optical diagram of a demultiplexer accordingto the present invention.

FIG. 2 shows a cross-section drawing of a demultiplexer according to thepresent invention.

FIGS. 3A and 3B show three-dimensional drawing of a demultiplexeraccording to the present invention.

FIG. 4 shows a projection of spatially separated optical beams in ademultiplexer according to the present invention.

DETAILED DESCRIPTION

FIG. 1A shows an optical ray diagram of a demultiplexer system 100according to the present invention. Demultiplexer system 100 includes atransmission diffraction grating 110. Diffraction grating 110 is formedon the surface of a plastic part 150 and, therefore, has an internalsurface and an external surface. Diffraction grating 110 can bepositioned so that light beam 120 is incident substantially normal tothe internal surface of grating 110. Transmission grating 110 includesruled lines 140 which can be blazed in order to preferentially couplelight into selected orders of diffraction peaks. Angularly separatedbeams 122 (including individual beams 122-1 through 122-N) are emittedfrom the external surface of diffraction grating 110.

FIG. 1C shows the external surface (opposite the internal surface onwhich light beam 120 is incident) of diffraction grating 110. Theexternal surface of diffraction grating 110 as shown in FIG. 1C can haveparallel formed ruled lines 140 which are formed in plastic part 150.Ruled lines 140 can be blazed in order to preferentially couple lightinto selected orders of diffraction peaks, for example into the firstorder diffraction peak. Rulings 140 can be formed on the surface ofplastic part 150 by forming trenches in the surface of plastic part 150with appropriately shaped side walls. Although the particular example oftransmission grating 110 of FIG. 1C has a one-dimensional ruled grating,other transmission gratings, for example a two-dimensionally ruledgrating, can also be utilized.

The diffraction pattern resulting from grating 110 includes zero-thorder (n=0), first order (n=1) and higher order diffraction peaks. Asshown in FIG. 1B, all but the zero-th order pattern separates incomingbeam 120 into individually separate beams 122-1 through 122-N based onthe individual wavelengths of light in WDM beam 120. Beam 120, then, isdemultiplexed into its separate wavelengths, for example, within thefirst-order (n=1) diffraction peak 122. In some embodiments, diffractiongrating 110 can be blazed to preferentially couple light from WDM beam120 into the first order diffraction peak of grating 110.

As shown in FIG. 1B, beam 120 can be separated with a larger angularseparation in higher order peaks, however, the intensity of lightcoupled into those diffraction peaks is usually much less than theintensity coupled into the zero-th order peak and the first-order peak.In some embodiments, grating 110 has rulings 140 blazed appropriately tocouple substantially all of beam 120 into the first order diffractionpeak (e.g., about 95%).

In one example, diffraction grating 110 can have 2000 lines/mm over 1.5mm. Additionally, beam 120 can include optical channels at wavelengthsof 1275 nm, 1300 nm, 1325 nm, and 1350 nm. In some embodiments, moldedsurface relief gratings are capable of achieving about 1 μm trenchdepths and greater than about 1 μm grating pitch. In accordance withsome embodiments of the present invention, a trench depth of about 0.5μm and a pitch of about 5 μm can be used. In some embodiments, thegrating can be an 8 level grating.

The first order diffraction beam 122, which includes individual beams122-1 through 122-N, is, then, angled at an angle θ from a normal to thesurface of transmission grating 110, where the angle θ represents thecentral angle of individual beams 122-1 through 122-N, θ₁ through θ_(N),respectively. The angle θ, then, can represents the average angularposition of individual beams 122-1 through 122-N from the normal, θ₁through θ_(N), respectively. In some embodiments, the angle θ representsthe average of the angular separation of the outside individual beams122-1 and 122-N, θ₁ and θ_(N) respectively.

Each of individual beams 122-1 through 122-N, separated by wavelengths,are distributed about the angle θ in a plane determined by the rulingsof grating 110. With a linearly ruled diffraction grating such as theexample of grating 110 shown in FIG. 1C, individual beams 122-1 through122-N lie in a plane defined by the normal direction to the surface ofgrating 110 and a direction perpendicular to rulings 140 of grating 110(denoted by angle (Φ=0 in the plane of diffraction grating 110). Ifgrating 110 is a two-dimensional grating (i.e., rulings 140 includerulings in two dimensions), then individual beams 122-1 through 122-Nmay be directed along a direction designated by radial angle Φ in theplane of the surface of grating 110 and at an azimuthal angle θ from thenormal to the surface of grating 110.

In some embodiments, diffraction beam 122 is at an angle θ of about 45°and Φ being 0°. Furthermore, individual beams 122-1 through 122-N ofeach of the channels are angularly separated by about 2°. In an examplewith N=4 having wavelengths of λ₁=1275 nm, λ₂=1300 nm, λ₃=1325 nm,λ₄=1350 nm, and with transmission grating 110 having 2000 lines/mm, asdescribed above, each of individual beams 122-1 through 122-4 can beangled at an angle of θ₁ about 42°, θ₂ about 44°, θ₃ about 46°, and θ₄about 48°, respectively, from the normal direction to the surface oftransmission grating 110. Over a transmission distance of about 12 mm,individual beams 122-1 through 122-4 can then be separated by about 250μm.

With a linearly ruled grating, each of individual beams 122-1 through122-N are directed in a direction perpendicular to the rulings 140 oftransmission grating 110. The angular separation allows each of theseparate beams in diffraction beam 122 to be incident on a correspondingone of detectors 125-1 through 125-N in a detector array 125. Therefore,the detectors in detector array 125 provide electrical signals inresponse to optical signals at each of the individual wavelengths of theindividual beams in diffraction beam 122. In embodiments as discussedabove with 250 μm separations over the transmission length (i.e., thedistance between the top surface of diffraction grating 110 and thecollection surface of detector array 125) of demultiplexer 100, then,the pitch of detectors 125-1 through 125-4 is about 250 μm. Althoughdetector array 125 having the appropriate pitch is relativelyinexpensive in increased cost over less compact systems, the greatlyincreased expense of forming optical filters of appropriate sizes toprovide a pitch of 250 μm is not required.

FIG. 2 shows a cross-sectional drawing of a demultiplexer 250 accordingto the present invention. Demultiplexer 250 includes a demultiplexermodule 200. Demultiplexer module 200 can be described as includingindividual sections 201, 202 and 203. Sections 201, 202 and 203 areintegrally formed as a single piece to form demultiplexer module 200.Section 201 receives WDM beam 120 and directs beam 120 onto the internalsurface of diffraction grating 110. Diffraction grating 110, then, isformed on an external surface of section 201. Section 203 receivesindividual beams 122-1 through 122-N and directs individual beams 122-1through 122-N onto the surface of detector array 125, couples them intoindividual optical fibers, or receives light from optical sources (ifmodule 200 is included in a multiplexer system). Section 202 providesstructural support and spatially aligns sections 2θ₁ and 203. Sections201, 202, and 203 are integrally formed by, for example, injectionmolding with an optically transparent material. Examples of opticallytransparent materials that can be utilized in forming demultiplexer 200include Ultem® or Lexan®, both produced by General Electric Corporation.

In some embodiments, section 201 of demultiplexer 200 can includecollimating lens 220 and reflection surface 210 along with transmissiongrating 110. Collimating lens 220 can be positioned relative to anoptical fiber 240, which can carry WDM multiplexed beam 120, so as tocollimate the light beam emitted from optical fiber 240. In someembodiments, lens 220 can be an aspheric lens that collimates light froma 0.3 NA fiber output.

Collimated light beam 120 can be reflected from reflector 210 anddirected onto the back surface of diffraction grating 110. Reflector 210can be formed as a total internal reflector or can include a reflectivebacking such as, for example, a gold or silver film. Further, reflection210, as a total internal reflector, can by coated on the surfaceopposite the surface on which beam 120 is incident (i.e., the surfaceexposed external to diffraction mounting 200), with thin films in orderto enhance reflection of beam 120.

Diffraction grating 110 can be formed in the surface of section 201during the injection molding process. In some embodiments, diffractiongrating 110 can be a one-dimensional ruled grating having evenly spacedtrenches formed on the external surface of section 201 of demultiplexermodule 200. In other embodiments, diffraction grating 110 can be atwo-dimensional ruled grating, as discussed above. In some embodiments,diffraction grating 110 can be formed after injection molding otherportions of demultiplexer module 200 by, for example, a number ofetching processes. In the case where grating 110 is fabricated as aseparate piece, grating 110 can be injection molded in atemperature-controlled environment and in a mold containing the reliefmicrostructure to be replicated, referred to as the master. The mastercan be fabricated using lithographic fabrication of multiple levels ofstructures by using multiple steps of photoresist application,lithography and etching.

Lens and detector section 203 can include integrally formed focusinglens 222. Lens and detector section 203 may also include support 224formed surrounding lens 222. Support 224, within which lens 222 can beformed, is integrally formed with support section 202. A detector array232 (which includes detectors 232-1 through 232-N) mounted on arrayholder 231 can be positioned on support 224 and aligned such thatindividually separated beams of light 122-1 through 122-N are incidentupon detectors 125-1 through 125-N, respectively. Each of detectors125-1 through 125-N can provide electrical signals on electricalconnections 232-1 through 232-N, respectively. Alternatively, each ofdetectors 125-1 through 125-N may be optical fibers so that light fromeach of individual beams 122-1 through 122-N is coupled intocorresponding optical fibers.

Optical detectors 125-1 through 125-N of detector array 125 can be anyoptical detector device, including GaAs, Si, or InGaAs basedphotodetectors in either discrete or array form, optical fibers coupledto receive light corresponding to individual channels, or opticalsources if demultiplexer 200 is utilized as a multiplexer instead of ademultiplexer. Optical sources can include photodiodes, vertical cavitysurface-emitting lasers (VCSELS), or light carrying optical fiber.

Support 202 connects section 201 with 203 and is integrally formed withsection 201 and 203. Support 202 is shaped so that section 203 isaligned with individual beams 122-1 through 122-N. In some embodiments,support 201 is a rectangular member with long axis angled at angle θrelative to a normal to the surface of grating 110.

In operation, light beam 120 from optical fiber 240 is collimated bylens 220 and directed onto transmission grating 110 by reflectivesurface 210. In some embodiments, light beam 120 is incident normally onan internal surface (i.e., internally to section 201) of transmissiongrating 110. Separated beam 122, including individual beams 122-1through 122-N, is emitted from the opposite surface (i.e., the externalsurface) of transmission grating 110 at an angle θ from the normal tothe opposite surface of transmission grating 110, as discussed above.Individual beams 122-1 through 122-N propagate outside of demultiplexermounting 200 and reenters demultiplexer mounting 200 substantiallynormally at surface 226 of lens and detector portion 203. Surface 226,in some embodiments, can be coated with an anti-reflective film toincrease the efficiency of demultiplexer 250. Lens 222 formed oppositesurface 226 in lens and detector portion 203 focuses individual beams122-1 through 122-N onto detectors 125-1 through 125-N, respectively, ofdetector array 125.

Demultiplexer module 200 can be formed as a single unit by an injectionmolding process, eliminating the steps of aligning through the opticalsystem. The remaining alignment problem is reduced to insuring thatfiber 240 is appropriately aligned with lens 220 and detector array 124is appropriately aligned with lens 222. In some embodiments, a barrelsupport can be integrally formed with demultiplexer module 200 in orderto align fiber 240. Further, alignment of support 231, where detectorarray 125 is mounted, can be accomplished through providing appropriateguide grooves in support 224 of demultiplexer module 200.

Further, demultiplexer module 200 provides a compact package. In someembodiments, demultiplexer module 200 can have a length L₁ of about 12mm and a width L₂ of about 12.5 mm. Further, demultiplexer module 200can have a cylindrical or rectangular cross section with width (ordiameter) of about 3 mm. In some embodiments, where the WDM gridincludes wavelengths of 1275 nm, 1300 nm, 1325 nm, and 1350 nm, thenlens 220 can be an aspherical lens with characteristics such as a radiusof curvature equal to 500 μm and a conic constant equal to −2.5 and lens222 can have characteristics such as a radius of curvature equal to 300μm and a conic constant equal to −2.5.

FIG. 3A shows a three-dimensional view of an embodiment of demultiplexermodule 200 according to the present invention. Demultiplexer module 200includes sections 201, 202 and 203, as discussed above. Further, a post310 can be integrally formed with section 201. In some embodiments,barrel 320 is removably affixed to part 310 and operates as a guide andpositioner for optical fiber 240. Barrel 320 can be positioned onsupport 326. Support 326 can be a ring which either protrudes or isformed as a trench with which barrel 320 mates. Barrel 320 can also beepoxied to part 310 on support 326 in order to provide better structuralintegrity to demultiplexer 250. In some embodiments, barrel 320 can beintegrally formed with demultiplexer module 200 by injection molding.Barrel 320 includes fiber access 322 with fiber stop 324. In operation,when optical fiber 240 is inserted into fiber access 322 to fiber stop324, optical fiber 240 is aligned with lens 220.

In some embodiments, fiber access 322 of barrel 320 has a depth of about4 mm before encountering fiber stop 324. The end of inserted fiber 240,which is flush with fiber stop 324, can then be separated by a distanceof about 1 mm from the vertex of lens 220. Fiber access 322 can have adiameter of about 2.5 mm in order to accept single mode, multi-mode,silica, or plastic optical fibers. In some embodiments, the outerdiameter of barrel 320 can be about 4 mm. Barrel 320 can be integrallymolded with demultiplexer module 200 or can be detachably coupled topost 310 which is integrally molded with demultiplexer module 200.

FIG. 3B shows a three-dimensional drawing of a demultiplexer module 200according to the present invention where transmission grating 110 isshown rather than reflection surface 210, as is shown in FIG. 3A.

FIG. 4 shows a beam profile at detector array 125 of an N=4 embodimentof a demultiplexer 250 according to the present invention. Each ofindependent beams 122-1 through 122-4 of FIG. 4 is resultant from aparticular embodiment of demultiplexer module 200 utilized with a WDMsystem having λ₁=1275 nm, λ₂=1300 nm, λ₃=1325 nm, and λ₄=1350 nm. FromFIG. 2, demultiplexer module 200 has length L₁=0.12 mm, width L₂=12.5mm, and thickness L₃=6 mm (see FIG. 3A). Furthermore, grating 110 ischaracterized as having 2000 lines/mm resulting in θ being about 45° (θ₁about 42°, θ₂ about 44°, θ₃ about 46°, θ₄ about 48°). Lens 220 ischaracterized as having a radius of curvature equal to 500 μm and aconic constant equal to −2.5 and lens 222 is characterized as having aradius of curvature equal to 300 μm and a conic constant equal to −2.5.The surface of detector array 125 is positioned a distance d of about500 μm from the base of lens 222 of section 203. Under those conditions,individual beams 122-1 through 122-4 are well resolved with separationsbetween beams 122-1 and 122-2, between beams 122-2 and 122-3, andbetween beams 122-3 and 122-4 (separations S₁, S₂, and S₃, respectively)of about 250 μm.

The embodiments described above are exemplary only and are not intendedto be limiting. One skilled in the art will recognize variations thatare intended to be within the spirit and scope of this disclosure. Assuch, the invention is limited only by the following claims.

1-16. (canceled)
 17. A method of forming a demultiplexer module,comprising: injection molding a part having a first section, a secondsection, and a third section, wherein a diffraction grating is formed inthe first section, the module is formed so that the third section ispositioned to receive light from the diffraction grating, and the secondsection is formed to support the third section relative to the firstsection.
 18. The method of claim 17, wherein injection molding includesforming a collimating lens in the first section.
 19. The method of claim17, wherein injection molding includes forming a reflective surface inthe first section.
 20. The method of claim 17, wherein injection moldingincludes forming a focusing lens in the third section.
 21. The method ofclaim 20, wherein injection molding includes forming a support aroundthe focusing lens in the third section.
 22. The method of claim 19,further including providing a coating to an outside surface of the firstsection opposite the reflective surface.
 23. (canceled)